There are many situations encountered in the abrasive machining of workpieces in which the quality of the finished surface may vary due to variations in the grindability of the unfinished surfaces from one workpiece to another. An outstanding example of a situation in which this problem is encountered is in the grinding of the internal bore of a hydraulic valve lifter for use in an internal combustion engine. To perform this grinding operation, it is necessary to use a long wheel, which means that a considerable amount of spindle deflection takes place. It would be possible by well-known means to compensate for this deflection to prevent the bore from being ground with an undesired taper, if the grindability of the bores were the same in all workpieces. However, in the case of the hydraulic valve lifter, the workpiece must be heat-treated prior to the grinding operation. The heat treatment (or, in some cases, the basic material or previous operations) causes the grindability to vary from workpiece to workpiece. These differences in grindability from one workpiece to the next cause the spindle and the abrasive wheel to deflect by different amounts and to produce different amounts of taper in the internal bore of the valve lifter. Now, it so happens that taper in the internal bore of a hydraulic valve lifter is particularly undesirable, because, if the taper exceeds a small tolerance, noisy operation of the engine results.
It is the usual practice in grinding to provide a "sparkout" period at the end of a grinding cycle. This is accomplished by terminating cross-feed allowing the grinding to continue under the impetus of the deflection of the spindle, which deflection can be appreciable, particularly in an internal grinding machine. Such a sparkout period relieves the workpiece surface of any taper that may be present due to spindle bending and also provides for a better surface finish because of the light final pressure between the wheel and the workpiece.
As the demands on grinding machines increase for smaller tolerance on taper, size, and finish, the conventional sparkout leaves something to be desired. First of all, final gage size usually terminates the sparkout. The amount of taper in the workpiece bore (for internal grinding) depends on the amount of deflection left in the spindle at the final size point, and this depends, in turn, on the amount of deflection at the beginning of sparkout. As has been stated, this starting deflection varies from workpiece to workpiece, so that the final taper tends to vary also. If one attempts to overcome this deficiency by allowing the wheel to grind until deflection is entirely removed from the spindle, the grinding cycle becomes very long, and this is unacceptable in the case of an expensive automatic grinding machine, because it increases the amount of labor cost, overhead, and capitalization applicable to each workpiece.
It has been found, furthermore, that the best surface finish is obtained when the finish grinding takes place with an appreciable force between the wheel and workpiece, rather than allowing the force to fall off a very small value, as is sometimes true in the conventional sparkout. Such an appreciable force is, however, substantially less than the amount used in "controlled force" grinding where the force used is as high as is feasible without destruction of the wheel. Nevertheless, many attempts have been made in the past to provide a relatively low, but, nevertheless, appreciable value of force during the finish period of the grinding cycle. Since the controlled force used in the roughing portions of the cycle are produced by using a hydraulic cylinder and by controlling the oil pressure and drain restriction for the cylinder, it was natural to try to maintain this low-value force by this means. However, it proved to be unfeasible to hold the forces constant within the required limits, because of the conduits, grinding swarf, guards, etc. It was also suggested that the low-value force could be obtained by swivelling the wheelhead, but experiment proved that the amount of swivel necessary to take care of the deflection difference between finishing with a large wheel and finishing with a small wheel is infinitesimally small, i.e., in the order of magnitude of the deflection resulting from a force change of one pound. Maintaining forces within such low values has proved to be impossible, especially under shop conditions. One method that has been suggested to overcome these problems is shown and described in the patent of Ware et al U.S. Pat. No. 3,535,828 which issued Oct. 27, 1970. In that case sparkout takes place as usual without any crossfeed. A control, including a device for measuring the load on the wheelhead motor, is connected to the feed mechanism to bring about a dressing of the wheel if the rate of grinding takes place at too slow a speed. The speed of spark removal during sparkout is measured by starting a timer at the same time that sparkout starts and if the power meter does not show that the power to the wheel has dropped to a predetermined level before the timer times out, then this is an indication that the wheel is dull and the machine is operated to dress the wheel. The wheel is then returned to finish the sparkout. An extension of that particular method is shown and described in the patent of Uhtenwoldt U.S. Pat. No. 3,703,054 which issued Nov. 21, 1972. In this last patent, a small increment of load is introduced into the machine every time the meter shows that the power has dropped to a predetermined level. In this way the grinding wheel approaches the final size, while still maintaining appreciable force between the wheel and the workpiece. In this way the size and taper reach the desired amounts without the grinding operation following into the extremely low load situation that is described above as being undesirable.
While these devices operate in a desirable manner, nevertheless, they sometimes result in an extremely long grinding cycle which is, of course, undesirable from a business point of view because of the high capital expense of the grinding machine. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a grinding machine which will permit one to maintain the quality of finished surface within a narrow range of tolerance despite variations in wheel size.
Another object of this invention is the provision of a grinding machine for grinding deep bores in workpieces while maintaining taper within a narrow range of tolerance despite substantial variation in hardness of the unfinished bore surface as well as the diameter of the abrasive wheel.
It is another object of the instant invention to provide a grinding machine having an operating cycle which reduces the variation of taper in finished surfaces of revolution to within a narrow range of tolerance despite substantial variations in unfinished workpiece surface quality from workpiece to workpiece.
It is a further object of the invention to provide a grinding machine for grinding bores without wide variation in taper in workpieces where the wheel length is large as compared to the diameter.
A still further object of this invention is the provision of a grinding machine which will operate in such a manner that, when a given workpiece arrives in the machine and has a surface grindability outside of a narrow range of tolerance, the abrasive wheel surface will not be affected in such a way as to produce poor surface quality in subsequently ground workpiece surfaces.
It is a still further object of the present invention to provide a grinding machine which operates in such a way that when excessive loads are encountered (which would break down the wheel surface due to a variation in grindability of a particular workpiece), the apparatus automatically compensates for breakdown in abrasive wheel surface, so that not only is the quality of subsequent workpieces provided for, but the particular workpiece itself is finished with a quality within tolerance.
Another object of the invention is the provision of a grinding machine which grinds at a high force during a roughing grind followed by operation at a low force finish grind and in which the point at which the shift from rough to finish grind takes place is adjusted to compensate for the change in size of the wheel from new to worn conditions.
Another object of the invention is the provision of a grinding machine in which the breakdown of abrasive wheel surface due to the unusual character of the particular workpiece will be compensated for during the grinding cycle involving that particular workpiece, because the breakdown is sensed early enough in the cycle for compensation to be made to the abrasive wheel so that the cycle can finish in the normal way.
Another object of the invention is the provision of a grinding machine having a grinding cycle with a first rough grind and a subsequent finish grind in which a breakdown of the abrasive wheel due to difficult grindability of a particular workpiece is compensated for by dressing after the rough grind so that the finish grind takes place in a normal manner.
It is, therefore, an outstanding object of the invention to provide a grinding machine in which sparkout is feasible even under close tolerances of size, taper, and surface finish.
A further object of the present invention is the provision of a grinding machine in which the taper in successive workpieces is maintained at a preselected value.
It is another object of the instant invention to provide a grinding machine in which good repeatability of taper and size is obtainable.
It is a further object of the invention to provide a grinding machine wherein short grinding cycles are possible without exceeding selected tolerances on taper and finish.
It is a still further object of the present invention to provide a grinding machine in which surface finish does not change due to the reduction in diameter of the abrasive wheel as it wears and is dressed from the large new wheel to the small worn wheel.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.